The present invention relates generally to air conditioning systems for regulating the temperature and humidity of ambient air, and more specifically to an efficient adsorbent composition and corrugated wheel medium for such a system.
Air conditioning systems include systems which regulate air temperature either by cooling or heating and regulate air humidity by humidifying or dehumidifying air. Conventional air conditioning systems typically use a heat-exchange medium like a refrigerant liquid which exits a compressor in a vapor phase at high temperature and pressure; passes through a heat exchanger, exiting in the liquid phase at moderate temperature and high pressure, and purging heat to the external environment; passes through a flow restriction, exiting as a mixed liquid and vapor at low temperature and pressure; enters a second heat exchanger to be gradually warmed by warm room air, cooling the room in the process; and finally returns to the compressor in the vapor phase at low temperature and pressure to start the process once again. However, the compressor is typically operated by electricity, which can be expensive, particularly in geographical areas characterized by severe climates or high costs for such service. Moreover, the use of conventional chlorofluorocarbons ("CFCs") like CFC-12 and hydrochlorofluorocarbons ("HCFCs") like HCFC-22 as refrigerant liquids has been shown to have adverse effects on the ozone layer, and is being gradually phased out.
In the past, alternative air conditioning systems have been developed to regulate the temperature and humidity of ambient air in an efficient manner without HCFCs or CFCs. Some systems even are powered by non-electric sources such as natural gas, which can be plentiful and inexpensive in some geographical areas.
An early system for drying air at ambient conditions was described in U.S. Pat. No. 2,926,502 issued to Munters. The Munters system utilized an adsorbent composition which dried the air by adsorbing the water out of the air. Such adsorbents which adsorb water are specifically referred to as desiccants.
In the Munters system, ambient air was passed through a rotating wheel made from corrugated paper formed from wool fibers or fibers of other highly hygroscopic materials to adsorb water moisture contained therein. A layer of silica gel could be added to the surface of the hygroscopic paper sheet to enhance the adsorption of water moisture from the ambient air. The process air exited the rotating wheel at a higher temperature due to the heat of adsorption, but was subsequently cooled by passing it through a heat exchanger countercurrent to a secondary air stream. Prior to entry into a room, the process air could be further cooled by adding moisture thereto, which also served to bring the humidity into the comfort zone. At the same time, the hotter secondary air stream was heated further by an electric heater, and used to regenerate the wheel by drawing off the moisture adsorbed therein. This open-cycle system which used one region of a rotating adsorbent wheel to adsorb moisture from ambient air, while using a secondary air stream to simultaneously remove moisture from another region of the wheel became known as the Munters Environmental Control ("MEC") System.
Many efforts have been made since the 1960's to improve the MEC system to meet general market needs, including the needs of the light commercial and residential sectors. One such improvement involved the substitution of a natural gas burner for Munter's electric heater. However, much of the development efforts have focused upon the search for an improved desiccant composition to provide enhanced adsorption/regeneration characteristics, to improve the thermal coefficient of performance ("COP") of the system, reducing operating costs, and to reduce the required equipment size through improved system cooling capacities, lowering the initial capital costs.
Hydrateable salt solutions like LiCl and LiBr have been applied to adsorbent wheels, as shown in U.S. Pat. No. 3,251,402 issued to Glav; U.S. Pat. No. 4,594,860 issued to Coellner et al.; and U.S. Pat. No. 4,729,774 issued to Cohen et al. However, these desiccant materials tend to deliquesce at high humidities and during periods of non-use, thereby causing the salts to "weep" from the wheel structure, and resulting in desiccant losses that greatly reduce system COP and in corrosion of the equipment. While the addition of a porous solid desiccant such as silica gel reduces this weeping phenomenon, it does not eliminate it.
Solid desiccants have also been used as the active component for adsorbent wheels. The COP and the cooling capacity of an adsorption system may be enhanced through improved adsorption performance, and higher than normal regeneration temperatures. Moreover, the total equilibrium capacity of the desiccant is important. While the rotational velocity of the adsorbent wheel can be increased to compensate for low adsorption capacity, if the wheel is turned too quickly and its heat capacity is too high, then excessive heat can be transferred to the adsorption side of the wheel, thereby reducing its COP.
An ideal desiccant for air conditioning applications is one for which, during the adsorption cycle, the moisture front will be sharp enough to be contained, and, during the regeneration cycle, will efficiently yield the adsorbed water without a breakthrough of the temperature front. Modeling of open-cycle adsorption systems (i.e., Collier et al. 1986) has indicated the need for a solid desiccant exhibiting a particular concave-down isotherm shape. The isotherm shape can be derived by the following formula: ##EQU1## where, RH=relative humidity; and
R=separation factor.
The isotherm is derived by plotting normalized water loading (water loading divided by water loading at 60% RH) as a function of relative humidity.
As shown in FIG. 1, water adsorption Isotherm X with a separation factor R of 1.0 is linear in shape. Water adsorption isotherm Z with a separation factor R of 0.01 has a steep concaved-down shape similar to a Brunauer Type 1 isotherm as also shown in FIG. 1. Water adsorption Isotherm Y with a separation factor R of 0.1, falls between the linear and Type 1 isotherms, and is referred to more commonly as a Type 1M moderate isotherm. (Isotherm Y was derived, assuming a water loading of 25% at 60% RH.) An adsorbent wheel which exhibits Type 1M isothermal behavior would be desirable, but adsorbent wheels exhibiting such characteristics have not until now been possible.
Alumina has been used as a solid desiccant in systems disclosed by U.S. Pat. No. 4,398,927 issued to Asher et al., and U.S. Pat. No. 4,875,520 issued to Steele et al., while silica gel has been suggested by Steele, Cohen, Asher, Munters, U.S. Pat. No. 4,341,539 issued to Gidaspow et al., and U.S. Pat. No. 4,911,775 issued to Kuma et al. Asher and U.S. Pat. No. 4,871,607 issued to Kuma et al. have also suggested the combination of silica and alumina in an absorption system. However, it has been found that these particular materials have nearly linear water adsorption isotherms (i.e., R=1.0), which do not yield optimum adsorption performance in air conditioning systems, because the adsorption moisture fronts passing through the adsorbent wheel are too broad, and the early moisture breakthrough results in a low COP for the system.
Natural zeolites and synthetic molecular sieves also have been used as solid desiccant components. For example, U.S. Patent No. 4,886,769 issued to Kuma et al., and No. 4,769,053 issued to Fischer, Jr. have disclosed use of 4-A or 3-A zeolites, while U.S. Pat. No. 4,595,403 issued to Sago et al. and U.S. Pat. Nos. 3,844,737, 4,012,206, and 4,134,743 issued to Macriss et al., teach the use of a 13-X zeolite. However, such zeolites or molecular sieve compositions commonly exhibit isotherms having a separation factor that is too low. Isotherms with low separation factors indicate that the desiccant adsorbs water too strongly, thereby making it difficult to subsequently desorb the water during a regeneration cycle. This results in breakthrough of the thermal wave which reduces the COP for the system.
Support material for the desiccant composition in the wheel has been made from many different types of fibers using a conventional paper-making process. Munters teaches the use of wool fibers, while Glav discloses the use of cellulose fibers. Glav and Macriss teach the use of asbestos. The paper may be corrugated to form a fluted layer, which is then laminated to a flat layer to form channels through which the ambient air passes. It may then be spirally wrapped around a central hub to form the wheel.
The desiccant has generally been added to the surface of the previously formed paper (e.g., Glav, Coellner, Sago, Fischer, Kuma and Steele). However, such wheels have been limited in use by the temperature limitations of cellulosic fibers or environmental limitations of asbestos.
Temperature-resistant materials like glass or ceramic fibers have been used as supports for desiccants, as illustrated by Sago and Kuma. However, these materials must be formed into fluted structures before the desiccant is loaded into the structure, because loaded glass and ceramic fibers are vulnerable to brittle fracture during the corrugation process. Subsequent saturation of the corrugated glass fiber matrix with a desiccant slurry or solution results in relatively low total loadings of desiccant (&lt;60%) even if multiple impregnations are used.
Other desiccant support materials that have been tried previously include metal strips (e.g., U.S. Pat. No. 4,172,164 issued to Meyer et al., and Fischer), and plastic matrices, such as nylon (e.g., Steele, Gidaspow, and Macriss). They have proved insufficient where high levels of moisture adsorption and desorption are required.
Since solid desiccants are most readily available in powdered or granular forms, it is necessary to bond the desiccant to or incorporate it into the wheel matrix. Since the physical integrity of the structure is a key concern for achieving extended wheel life, the materials must remain bonded to the structure after long-term operation of the wheel at higher-than-normal regeneration temperatures (e.g., up to 200.degree. C.).
Although inactive components are used for the purpose of bonding, there are limits on the amount of these inactive components which may be used. Excessive use of organic components which can withstand regeneration temperatures can result in dilution of the adsorption system and blockage of the active desiccant pores, which results in reduced moisture adsorption of the wheel. Excessive amounts of inorganic binders can result in formation of brittle bonds that can be detrimental to wheel-forming operations.